For drilling an oil field or a natural gas field, oil country tubular goods are used. In proportion to the depth of a well, multiple steel pipes are connected to form the oil country tubular goods. Coupling the steel pipes is conducted by fastening threaded connections for pipe and tube formed at end portions of the steel pipes. The oil country tubular goods are drawn up and loosened for inspection or the like, and after the inspection, the oil country tubular goods are fastened again and reused.
A threaded connection for pipe or tube includes a pin and a box. The pin includes an external thread portion and an unthreaded metal contact portion formed on an outer peripheral surface of a front end portion of a steel pipe. The box includes an internal thread portion and an unthreaded metal contact portion formed on an inner peripheral surface of a front end portion of a steel pipe. The thread portions and the unthreaded metal contact portions of the pin and the box repeatedly receive strong friction in fastening (making-up) and loosening (breaking-out) steel pipes. If these portions do not have a sufficient durability against friction, galling (unrepairable seizure) occurs when fastening and loosening are repeated. Therefore, a sufficient durability against friction, namely, excellent galling resistance is needed for a threaded connection for pipe or tube.
In conventional practices, to improve galling resistance, a compound grease called dope, containing heavy metals, has been used. By applying the compound grease on the surface of a threaded connection for pipe or tube, the galling resistance of the threaded connection for pipe or tube can be improved. However, heavy metals such as Pb, Zn, and Cu contained in the compound grease may exert an influence on the environment. For this reason, the development of a threaded connection for pipe or tube not using compound grease has been desired.
Japanese Patent Application Publication No. 8-105582 (Patent Literature 1), International Application Publication No. WO1996/10710 (Patent Literature 2) and Japanese Patent Application Publication No. 2008-215473 (Patent Literature 3) propose threaded connections for pipe or tube excellent in galling resistance even without compound grease.
A threaded connection for pipe or tube described in Patent Literature 1 includes Cr content of 10% or more, and on the contact surface of a box or a pin, a nitrided layer with a thickness of 1 to 20 m and a three-layer coating are formed, wherein the three-layer coating layer is comprising of an underlying processing layer, a manganese phosphate-based chemical treated layer with a thickness of 5 to 25 m, and a solid lubricant layer with a thickness of 10 to 45 μm. The underlying processing layer is chosen from a nitrided layer with a thickness of 1 to 20 μm, an iron plating layer or an iron alloy plating layer with a thickness of 0.5 to 15 μm. Thus, Patent Literature 1 describes that, even when the number of use of the steel pipe increase, it is possible to prevent occurrence of galling of a threaded connection for pipe or tube for a long time.
On the contact surface of a box or a pin of a threaded connection for pipe or tube described in Patent Literature 2, a phosphoric chemical treatment coating layer or a nitrided layer, and a phosphoric chemical treatment coating layer are provided, and a resin coating layer is formed on the phosphoric chemical treatment coating layer, the resin coating layer being obtained by dispersing molybdenum disulfide or tungsten disulfide powder into a resin and mixing them. Furthermore, the thickness of the resin coating is equal to or larger than the thickness of the phosphoric chemical treatment coating layer. Patent Literature 2 describes that it is thereby possible to prevent the occurrence of galling in a threaded connection for a long time even if the number of repetitive uses of a steel pipe increases.
In a threaded connection for pipe or tube described in Patent Literature 3, the contact surface of at least one of a pin and a box includes a first plating layer consisting of a Cu—Zn alloy. Patent Literature 3 describes that this allows the threaded connection to have excellent leakage resistance and galling resistance and furthermore improves crevice corrosion in the case where a lubricating coating is formed on the plating layer.
To suppress galling in a threaded connection for pipe or tube, it is effective to form a plating layer containing a metal having a high hardness and a high melting point. For this reason, copper (Cu) plating or Cu-alloy plating is used in conventional practice. Cu has a high hardness and a high melting point. Therefore, Cu contained in the plating layer increases the hardness and the melting point of the plating layer as a whole. Consequently, the galling resistance of the threaded connection for pipe or tube is increased.